The simplest prior art method of a bidirectional data transmission over optical waveguides includes a separate optical waveguide for each direction of transmission. This method has proven disadvantageous based on the high cost for long-distance communications, due to the employment and laying of one optical waveguide for each direction of transmission.
Further, there is known in the art a wavelength division multiplexing method, wherein different wavelengths or frequencies, resp., are used for each direction of transmission. A wavelength .lambda..sub.1 =1,300 nm can be used, e.g., for the one direction, and .lambda..sub.2 =1,550 nm for the other direction. By using different wavelengths of the transmitting light, the transmission of both directions can be performed on a single optical waveguide. Each of the two transmitter/receiver units must include a wavelength-selective component (multiplexer), being used for the separation of the two wavelengths. If the difference of the two wavelengths .DELTA..lambda.=.lambda..sub.2 -.lambda..sub.1 is within the bandwidth of the sensitivity of the employed optical receivers, it is not necessary to discriminate between the two wavelengths in the optical receiver. It is disadvantageous, herein, that the wavelength-selective elements have to provide a high selectivity or attenuation, resp., such that only very good multiplexers having a high wavelength isolation can be used. These are very expensive. Moreover, proper communication over long distances will require an attenuation of at least 50 db between the two channels, in order to avoid cross-talk between the channels.
Another prior art method is the packet-switched data transmission, wherein, over a single optical waveguide, alternatingly for both directions, the data are transmitted in the form of packets. At the beginning and at the end of each packet, start or stop information for control and monitoring of the transmission is inserted. After the stop information being received and evaluated at the actual receiver side, the transmitter can, in the opposite direction of the transmission path, transmit its data as a packet being also provided with start and stop information. It has to be considered, therein, that a long pause is to be maintained between two packets, which has, due to the transmission time, to be at least as long as the transit time over the transmission path. It is disadvantageous, herein, that due to the long pauses, the transmission path cannot be utilized in an optimum manner. Moreover, the packet-switched transmission method requires an expensive clock recovery, large data rates and a complicated control system. Depending on the length of the packets, a large memory may also be necessary.